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My personal experience is that when using both a Pentax *ist DS (violent mirror slap) and Canon 40D (much less violent mirror slap) I can get blurred Moon images (1/125 sec and slower) as well as “double” stars on long exposure DSO images, where the bright stars have a fainter double just to the side, a ghost resulting from the vibration caused by mirror slap.
This has happened on a Vixen GPDX GEM mount and a Takahashi EM200 GEM mount with a 4” F9 refractor and Mewlon 180 F12 Cat despite having the ‘scopes carefully balanced and using a remote release or the software utility to fire the shutter.
As a result, I no longer risk ruining a shot so now always use mirror lock up. As Matty writes, a heavy, solid mount should minimise this or even prevent it happening.
Cheers
Dennis
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